AtUTr2 is an Arabidopsis thaliana nucleotide sugar transporter located in the Golgi apparatus capable of transporting UDP-galactose

Holistic insights from pollen omics: co-opting stress-responsive genes and ER-mediated proteostasis for male fertility

Sexual reproduction in flowering plants takes place without an aqueous environment. Sperm are carried by pollen through air to reach the female gametophyte, though the molecular basis underlying the protective strategy of the male gametophyte is poorly understood. Here we compared the published transcriptomes of Arabidopsis thaliana pollen, and of heat-responsive genes, and uncovered insights into how mature pollen (MP) tolerates desiccation, while developing and germinating pollen are vulnerable to heat stress. Germinating pollen expresses molecular chaperones or "heat shock proteins" in the absence of heat stress. Furthermore, pollen tubes that grew through pistils at basal temperature showed induction of the endoplasmic reticulum (ER) stress response, which is a characteristic of stressed vegetative tissues. Recent studies show MP contains mRNA-protein (mRNP) aggregates that resemble "stress" granules triggered by heat or other stresses to protect cells. Based on these observations, we postulate that mRNP particles are formed in maturing pollen in response to developmentally programmed dehydration. Dry pollen can withstand harsh conditions as it is dispersed in air. We propose that, when pollen lands on a compatible pistil and hydrates, mRNAs stored in particles are released, aided by molecular chaperones, to become translationally active. Pollen responds to osmotic, mechanical, oxidative, and peptide cues that promote ER-mediated proteostasis and membrane trafficking for tube growth and sperm discharge. Unlike vegetative tissues, pollen depends on stress-protection strategies for its normal development and function. Thus, heat stress during reproduction likely triggers changes that interfere with the normal pollen responses, thereby compromising male fertility. This holistic perspective provides a framework to understand the basis of heat-tolerant strains in the reproduction of crops.

The Arabidopsis thaliana nucleotide sugar transporter GONST2 is a functional homolog of GONST1

Glycosylinositolphosphorylceramides (GIPCs) are the predominant lipid in the outer leaflet of the plasma membrane. Characterized GIPC glycosylation mutants have severe or lethal plant phenotypes. However, the function of the glycosylation is unclear. Previously, we characterized Arabidopsis thaliana GONST1 and showed that it was a nucleotide sugar transporter which provides GDP-mannose for GIPC glycosylation. gonst1 has a severe growth phenotype, as well as a constitutive defense response. Here, we characterize a mutant in GONST1's closest homolog, GONST2. The gonst2-1 allele has a minor change to GIPC headgroup glycosylation. Like other reported GIPC glycosylation mutants, gonst1-1gonst2-1 has reduced cellulose, a cell wall polymer that is synthesized at the plasma membrane. The gonst2-1 allele has increased resistance to a biotrophic pathogen Golovinomyces orontii but not the necrotrophic pathogen Botrytis cinerea. Expression of GONST2 under the GONST1 promoter can rescue the gonst1 phenotype, indicating that GONST2 has a similar function to GONST1 in providing GDP-D-Man for GIPC mannosylation.

Biosynthesis and Transport of Nucleotide Sugars for Plant Hemicellulose

Hemicellulose is entangled with cellulose through hydrogen bonds and meanwhile acts as a bridge for the deposition of lignin monomer in the secondary wall. Therefore, hemicellulose plays a vital role in the utilization of cell wall biomass. Many advances in hemicellulose research have recently been made, and a large number of genes and their functions have been identified and verified. However, due to the diversity and complexity of hemicellulose, the biosynthesis and regulatory mechanisms are yet unknown. In this review, we summarized the types of plant hemicellulose, hemicellulose-specific nucleotide sugar substrates, key transporters, and biosynthesis pathways. This review will contribute to a better understanding of substrate-level regulation of hemicellulose synthesis.

New steps in mucilage biosynthesis revealed by analysis of the transcriptome of the UDP-rhamnose/UDP-galactose transporter 2 mutant

Upon imbibition, epidermal cells of Arabidopsis thaliana seeds release a mucilage formed mostly by pectic polysaccharides. The Arabidopsis mucilage is composed mainly of unbranched rhamnogalacturonan-I (RG-I), with low amounts of cellulose, homogalacturonan, and traces of xylan, xyloglucan, galactoglucomannan, and galactan. The pectin-rich composition of the mucilage and their simple extractability makes this structure a good candidate to study the biosynthesis of pectic polysaccharides and their modification. Here, we characterize the mucilage phenotype of a mutant in the UDP-rhamnose/galactose transporter 2 (URGT2), which exhibits a reduction in RG-I and also shows pleiotropic changes, suggesting the existence of compensation mechanisms triggered by the lack of URGT2. To gain an insight into the possible compensation mechanisms activated in the mutant, we performed a transcriptome analysis of developing seeds using RNA sequencing (RNA-seq). The results showed a significant misregulation of 3149 genes, 37 of them (out of the 75 genes described to date) encoding genes proposed to be involved in mucilage biosynthesis and/or its modification. The changes observed in urgt2 included the up-regulation of UAFT2, a UDP-arabinofuranose transporter, and UUAT3, a paralog of the UDP-uronic acid transporter UUAT1, suggesting that they play a role in mucilage biosynthesis. Mutants in both genes showed changes in mucilage composition and structure, confirming their participation in mucilage biosynthesis. Our results suggest that plants lacking a UDP-rhamnose/galactose transporter undergo important changes in gene expression, probably to compensate modifications in the plant cell wall due to the lack of a gene involved in its biosynthesis.

The GDP-mannose transporter gene (DoGMT) from Dendrobium officinale is critical for mannan biosynthesis in plant growth and development

Dendrobium officinale is a precious traditional Chinese medicinal herb because it is abundant in mannose-containing polysaccharides (MCPs). GDP-mannose transporter (GMT), which translocates GDP-mannose into the Golgi lumen, is indispensable for the biosynthesis of MCPs. In this study, we found that the dominant polysaccharides in D. officinale were MCPs in a range of varieties and different physiological phases. After a positive correlation between the accumulation of mannose and the transcript levels of candidate GMT genes was found, three GMT genes (DoGMT1-3) were identified in D. officinale. DoGMT1, DoGMT2 and DoGMT3 exhibited the highest transcript level in stem that an organ for MCPs storage. All three DoGMT proteins were targeted to Golgi apparatus, and had a GDP binding domain (GXL/VNK) that was homologous to a specially characterized GMT protein GONST1 in Arabidopsis thaliana. Moreover, DoGMT1, DoGMT2 and DoGMT3 complemented a GDP-mannose transport-defective yeast mutant (vrg4-2), meanwhile they also demonstrated a higher GDP-mannose uptake activity. Therefore, we conclude that DoGMT1, DoGMT2 and DoGMT3 are able to transport GDP-mannose while the expression patterns of these genes correspond to the accumulation of MCPs in D. officinale. These findings support the importance of GMT genes from D. officinale in the biosynthesis of MCPs.

Expression of the human UDP-galactose transporter gene hUGT1 in tobacco plants' enhanced plant hardness

We reported previously that tobacco plants transformed with the human UDP-galactose transporter 1 gene (hUGT1) had enhanced growth, displayed characteristic traits, and had an increased proportion of galactose (hyper-galactosylation) in the cell wall matrix polysaccharides. Here, we report that hUGT1-transgenic plants have an enhanced hardness. As determined by breaking and bending tests, the leaves and stems of hUGT1-transgenic plants were harder than those of control plants. Transmission electron microscopy revealed that the cell walls of palisade cells in leaves, and those of cortex cells and xylem fibers in stems of hUGT1-transgenic plants, were thicker than those of control plants. The increased amounts of total cell wall materials extracted from the leaves and stems of hUGT1-transgenic plants supported the increased cell wall thickness. In addition, the cell walls of the hUGT1-transgenic plants showed an increased lignin contents, which was supported by the up-regulation of lignin biosynthetic genes. Thus, the heterologous expression of hUGT1 enhanced the accumulation of cell wall materials, which was accompanied by the increased lignin content, resulting in the increased hardness of the leaves and stems of hUGT1-trangenic plants. The enhanced accumulation of cell wall materials might be related to the hyper-galactosylation of cell wall matrix polysaccharides, most notably arabinogalactan, because of the enhanced UDP-galactose transport from the cytosol to the Golgi apparatus by hUGT1, as suggested in our previous report.

The inside and outside: topological issues in plant cell wall biosynthesis and the roles of nucleotide sugar transporters

The cell wall is a complex extracellular matrix composed primarily of polysaccharides. Noncellulosic polysaccharides, glycoproteins and proteoglycans are synthesized in the Golgi apparatus by glycosyltransferases (GTs), which use nucleotide sugars as donors to glycosylate nascent glycan and glycoprotein acceptors that are subsequently exported to the extracellular space. Many nucleotide sugars are synthesized in the cytosol, leading to a topological issue because the active sites of most GTs are located in the Golgi lumen. Nucleotide sugar transporters (NSTs) overcome this problem by translocating nucleoside diphosphate sugars from the cytosol into the lumen of the organelle. The structures of the cell wall components synthesized in the Golgi are diverse and complex; therefore, transporter activities are necessary so that the nucleotide sugars can provide substrates for the GTs. In this review, we describe the topology of reactions involved in polysaccharide biosynthesis in the Golgi and focus on the roles of NSTs as well as their impacts on cell wall structure when they are altered.

Overview of Nucleotide Sugar Transporter Gene Family Functions Across Multiple Species

Glycoproteins and glycolipids are crucial in a number of cellular processes, such as growth, development, and responses to external cues, among others. Polysaccharides, another class of sugar-containing molecules, also play important structural and signaling roles in the extracellular matrix. The additions of glycans to proteins and lipids, as well as polysaccharide synthesis, are processes that primarily occur in the Golgi apparatus, and the substrates used in this biosynthetic process are nucleotide sugars. These proteins, lipids, and polysaccharides are also modified by the addition of sulfate groups in the Golgi apparatus in a series of reactions where nucleotide sulfate is needed. The required nucleotide sugar substrates are mainly synthesized in the cytosol and transported into the Golgi apparatus by nucleotide sugar transporters (NSTs), which can additionally transport nucleotide sulfate. Due to the critical role of NSTs in eukaryotic organisms, any malfunction of these could change glycan and polysaccharide structures, thus affecting function and altering organism physiology. For example, mutations or deletion on NST genes lead to pathological conditions in humans or alter cell walls in plants. In recent years, many NSTs have been identified and functionally characterized, but several remain unanalyzed. This study examined existing information on functionally characterized NSTs and conducted a phylogenetic analysis of 257 NSTs predicted from nine animal and plant model species, as well as from protists and fungi. From this analysis, relationships between substrate specificity and the primary NST structure can be inferred, thereby advancing understandings of nucleotide sugar gene family functions across multiple species.

UDP-galactose transporter gene hUGT1 expression in tobacco plants leads to hyper-galactosylated cell wall components

We reported previously that tobacco plants transformed with the human UDP-galactose transporter 1 gene (hUGT1-transgenic plants) displayed morphological, architectural, and physiological alterations, such as enhanced growth, increased accumulation of chlorophyll and lignin, and a gibberellin-responsive phenotype. In the present study, we demonstrated that hUGT1 expression altered the monosaccharide composition of cell wall matrix polysaccharides, such as pectic and hemicellulosic polysaccharides, which are biosynthesized in the Golgi lumen. An analysis of the monosaccharide composition of the cell wall matrix polysaccharides revealed that the ratio of galactose to total monosaccharides was significantly elevated in the hemicellulose II and pectin fractions of hUGT1-transgenic plants compared with that of control plants. A hyper-galactosylated xyloglucan structure was detected in hemicellulose II using oligosaccharide mass profiling. These results indicated that, because of the enhanced UDP-galactose transport from the cytosol to the Golgi apparatus by hUGT1, galactose incorporation in the cell wall matrix polysaccharides increased. This increased galactose incorporation may have contributed to increased galactose tolerance in hUGT1-transgenic plants.

RNA-sequencing Oryza sativa transcriptome in response to herbicide isoprotruon and characterization of genes involved in IPU detoxification

The comprehensive analysis of transcriptome and UPLC-MS/MS in rice was performed to explore the regulatory mechanism of mRNA level and chemical metabolism in response to herbicide isoproturon.

Identification and Characterization of a Golgi-Localized UDP-Xylose Transporter Family from Arabidopsis

Most glycosylation reactions require activated glycosyl donors in the form of nucleotide sugars to drive processes such as posttranslational modifications and polysaccharide biosynthesis. Most plant cell wall polysaccharides are biosynthesized in the Golgi apparatus from cytosolic-derived nucleotide sugars, which are actively transferred into the Golgi lumen by nucleotide sugar transporters (NSTs). An exception is UDP-xylose, which is biosynthesized in both the cytosol and the Golgi lumen by a family of UDP-xylose synthases. The NST-based transport of UDP-xylose into the Golgi lumen would appear to be redundant. However, employing a recently developed approach, we identified three UDP-xylose transporters in the Arabidopsis thaliana NST family and designated them UDP-XYLOSE TRANSPORTER1 (UXT1) to UXT3. All three transporters localize to the Golgi apparatus, and UXT1 also localizes to the endoplasmic reticulum. Mutants in UXT1 exhibit ∼30% reduction in xylose in stem cell walls. These findings support the importance of the cytosolic UDP-xylose pool and UDP-xylose transporters in cell wall biosynthesis.

VvGONST-A and VvGONST-B are Golgi-localised GDP-sugar transporters in grapevine (Vitis vinifera L.)

Plant nucleotide-sugar transporters (NSTs) are responsible for the import of nucleotide-sugar substrates into the Golgi lumen, for subsequent use in glycosylation reactions. NSTs are specific for either GDP- or UDP-sugars, and almost all transporters studied to date have been isolated from Arabidopsis thaliana L. In order to determine the conservation of the import mechanism in other higher plant species, here we report the identification and characterisation of VvGONST-A and VvGONST-B from grapevine (Vitis vinifera L. cv. Thompson Seedless), which are the orthologues of the GDP-sugar transporters GONST3 and GONST4 in Arabidopsis. Both grapevine NSTs possess the molecular features characteristic of GDP-sugar transporters, including a GDP-binding domain (GXL/VNK) towards the C-terminal. VvGONST-A and VvGONST-B expression is highest at berry setting and decreases throughout berry development and ripening. Moreover, we show using green fluorescent protein (GFP) tagged versions and brefeldin A treatments, that both are localised in the Golgi apparatus. Additionally, in vitro transport assays after expression of both NSTs in tobacco leaves indicate that VvGONST-A and VvGONST-B are capable of transporting GDP-mannose and GDP-glucose, respectively, but not a range of other UDP- and GDP-sugars. The possible functions of these NSTs in glucomannan synthesis and/or glycosylation of sphingolipids are discussed.

Endoplasmic reticulum: Where nucleotide sugar transport meets cytokinin control mechanisms

The endoplasmic reticulum (ER) is a multifunctional eukaryotic organelle where the vast majority of secretory proteins are folded and assembled to achieve their correct tertiary structures. The lumen of the ER and Golgi apparatus also provides an environment for numerous glycosylation reactions essential for modifications of proteins and lipids, and for cell wall biosynthesis. These glycosylation reactions require a constant supply of cytosolically synthesized substrate precursors, nucleotide sugars, which are transported by a group of dedicated nucleotide sugar transporters (NST). Recently, we have reported on the identification of a novel ER-localized NST protein, ROCK1, which mediates the transport of UDP-linked acetylated hexosamines across the ER membrane in Arabidopsis. Interestingly, it has been demonstrated that the activity of ROCK1 is important for the regulation of cytokinin-degrading enzymes, cytokinin oxidases/dehydrogenases (CKX), in the ER and, thus, for cytokinin responses. In this addendum we will address the biochemical and cellular activity of the ROCK1 transporter and its phylogenetic relation to other NST proteins.

The Golgi localized bifunctional UDP-rhamnose/UDP-galactose transporter family of Arabidopsis

Plant cells are surrounded by a cell wall that plays a key role in plant growth, structural integrity, and defense. The cell wall is a complex and diverse structure that is mainly composed of polysaccharides. The majority of noncellulosic cell wall polysaccharides are produced in the Golgi apparatus from nucleotide sugars that are predominantly synthesized in the cytosol. The transport of these nucleotide sugars from the cytosol into the Golgi lumen is a critical process for cell wall biosynthesis and is mediated by a family of nucleotide sugar transporters (NSTs). Numerous studies have sought to characterize substrate-specific transport by NSTs; however, the availability of certain substrates and a lack of robust methods have proven problematic. Consequently, we have developed a novel approach that combines reconstitution of NSTs into liposomes and the subsequent assessment of nucleotide sugar uptake by mass spectrometry. To address the limitation of substrate availability, we also developed a two-step reaction for the enzymatic synthesis of UDP-l-rhamnose (Rha) by expressing the two active domains of the Arabidopsis UDP-l-Rha synthase. The liposome approach and the newly synthesized substrates were used to analyze a clade of Arabidopsis NSTs, resulting in the identification and characterization of six bifunctional UDP-l-Rha/UDP-d-galactose (Gal) transporters (URGTs). Further analysis of loss-of-function and overexpression plants for two of these URGTs supported their roles in the transport of UDP-l-Rha and UDP-d-Gal for matrix polysaccharide biosynthesis.

Carbohydrate metabolism changes in Prunus persica gummosis infected with Lasiodiplodia theobromae

Peach gummosis represents a significant global disease of stone fruit trees and a major disease in the south peach production area of the Yangtze River of China. In this study, the carbohydrate composition of peach shoots during infection by Lasiodiplodia theobromae was examined. The expression of genes related to metabolic enzymes was also investigated. Control wounded and noninoculated tissue, lesion tissue, and wounded and inoculated surrounding lesion tissue of peach shoots were analyzed. Soluble sugars, glucose, mannose, arabinose, and xylose significantly increased in inoculated tissues of peach shoots compared with control tissues at different times after inoculation. Accumulation of polysaccharides was also observed by section observation and periodic acid Schiff's reagent staining during infection. Analysis using quantitative reverse-transcription polymerase chain reaction revealed that the abundance of key transcripts on the synthesis pathway of uridine diphosphate (UDP)-D-glucuronate, UDP-D-galactose, and UDP-D-arabinose increased but the synthesis of L-galactose and guanosine diphosphate-L-galactose were inhibited. After inoculation, the transcript levels of sugar transport-related genes (namely, SUT, SOT, GMT, and UGT) was induced. These changes in sugar content and gene expression were directly associated with peach gum polysaccharide formation and may be responsible for the symptoms of peach gummosis.

Abnormal glycosphingolipid mannosylation triggers salicylic acid-mediated responses in Arabidopsis

The Arabidopsis thaliana protein GOLGI-LOCALIZED NUCLEOTIDE SUGAR TRANSPORTER (GONST1) has been previously identified as a GDP-d-mannose transporter. It has been hypothesized that GONST1 provides precursors for the synthesis of cell wall polysaccharides, such as glucomannan. Here, we show that in vitro GONST1 can transport all four plant GDP-sugars. However, gonst1 mutants have no reduction in glucomannan quantity and show no detectable alterations in other cell wall polysaccharides. By contrast, we show that a class of glycosylated sphingolipids (glycosylinositol phosphoceramides [GIPCs]) contains Man and that this mannosylation is affected in gonst1. GONST1 therefore is a Golgi GDP-sugar transporter that specifically supplies GDP-Man to the Golgi lumen for GIPC synthesis. gonst1 plants have a dwarfed phenotype and a constitutive hypersensitive response with elevated salicylic acid levels. This suggests an unexpected role for GIPC sugar decorations in sphingolipid function and plant defense signaling. Additionally, we discuss these data in the context of substrate channeling within the Golgi.

Arabidopsis thaliana AtUTr7 encodes a golgi-localized UDP-glucose/UDP-galactose transporter that affects lateral root emergence

Nucleotide sugar transporters (NSTs) are antiporters comprising a gene family that plays a fundamental role in the biosynthesis of complex cell wall polysaccharides and glycoproteins in plants. However, due to the limited number of related mutants that have observable phenotypes, the biological function(s) of most NSTs in cell wall biosynthesis and assembly have remained elusive. Here, we report the characterization of AtUTr7 from Arabidopsis (Arabidopsis thaliana (L.) Heynh.), which is homologous to multi-specific UDP-sugar transporters from Drosophila melanogaster, humans, and Caenorhabditis elegans. We show that AtUTr7 possesses the common structural characteristics conserved among NSTs. Using a green fluorescent protein (GFP) tagged version, we demonstrate that AtUTr7 is localized in the Golgi apparatus. We also show that AtUTr7 is widely expressed, especially in the roots and in specific floral organs. Additionally, the results of an in vitro nucleotide sugar transport assay carried out with a tobacco and a yeast expression system suggest that AtUTr7 is capable of transferring UDP-Gal and UDP-Glc, but not a range of other UDP- and GDP-sugars, into the Golgi lumen. Mutants lacking expression of AtUTr7 exhibited an early proliferation of lateral roots as well as distorted root hairs when cultivated at high sucrose concentrations. Furthermore, the distribution of homogalacturonan with a low degree of methyl esterification differed in lateral root tips of the mutant compared to wild-type plants, although additional analytical procedures revealed no further differences in the composition of the root cell walls. This evidence suggests that the transport of UDP-Gal and UDP-Glc into the Golgi under conditions of high root biomass production plays a role in lateral root and root hair development.

Golgi-localized UDP-glucose transporter is required for cell wall integrity in rice

Cell wall-related nucleotide sugar transporters (NSTs) theoretically supply the cytosolic nucleotide sugars for glycosyltransferases (GTs) to carry out ploysaccharide synthesis and modification in the Golgi apparatus. However, the regulation of cell wall synthesis by NSTs remains undescribed. Recently, we have reported the functional characterization of Oryza sativa nucleotide sugar transport (Osnst1) mutant and its corresponding gene. OsNST1/BC14 is localized in the Golgi apparatus and transports UDP-glucose. This mutant provides us with a unique opportunity for evaluation of its abroad impacts on cell wall structure and components. We previously examined cell wall composition of bc14 and wild type plants. Here, the spatial distribution of these cell wall alterations was analyzed by immunolabeling approach. Analysis of the sugar yield in different cell wall fractions indicated that this mutation improves the extractability of cell wall components. Field emission scanning electron microscopy further showed that the orientation of microfibrils in bc14 is irregular when compared to that in wild type. Therefore, this UDP-glucose transporter, making substrates available for polysaccharide biosynthesis, plays a critical role in maintaining cell wall integrity.

Golgi nucleotide sugar transporter modulates cell wall biosynthesis and plant growth in rice

Golgi-localized nucleotide sugar transporters (NSTs) are considered essential for the biosynthesis of wall polysaccharides and glycoproteins based on their characteristic transport of a large number of nucleotide sugars to the Golgi lumen. The lack of NST mutants in plants has prevented evaluation of this hypothesis in plants. A previously undescribed Golgi NST mutant, brittle culm14 (bc14), displays reduced mechanical strength caused by decreased cellulose content and altered wall structure, and exhibits abnormalities in plant development. Map-based cloning revealed that all of the observed mutant phenotypes result from a missense mutation in a putative NST gene, Oryza sativa Nucleotide Sugar Transporter1 (OsNST1). OsNST1 was identified as a Golgi-localized transporter by analysis of a fluorescence-tagged OsNST1 expressed in rice protoplast cells and demonstration of UDP-glucose transport activity via uptake assays in yeast. Compositional sugar analyses in total and fractionated wall residues of wild-type and bc14 culms showed a deficiency in the synthesis of glucoconjugated polysaccharides in bc14, indicating that OsNST1 supplies the glucosyl substrate for the formation of matrix polysaccharides, and thereby modulates cellulose biosynthesis. OsNST1 is ubiquitously expressed, with high expression in mechanical tissues. The inferior mechanical strength and abnormal development of bc14 plants suggest that OsNST1 has pleiotropic effects on cell wall biosynthesis and plant growth. Identification of OsNST1 has improved our understanding of how cell wall polysaccharide synthesis is regulated by Golgi NSTs in plants.

Engineering of N. benthamiana L. plants for production of N-acetylgalactosamine-glycosylated proteins--towards development of a plant-based platform for production of protein therapeutics with mucin type O-glycosylation

BACKGROUND

Mucin type O-glycosylation is one of the most common types of post-translational modifications that impacts stability and biological functions of many mammalian proteins. A large family of UDP-GalNAc polypeptide:N-acetyl-α-galactosaminyltransferases (GalNAc-Ts) catalyzes the first step of mucin type O-glycosylation by transferring GalNAc to serine and/or threonine residues of acceptor polypeptides. Plants do not have the enzyme machinery to perform this process, thus restricting their use as bioreactors for production of recombinant therapeutic proteins.

RESULTS

The present study demonstrates that an isoform of the human GalNAc-Ts family, GalNAc-T2, retains its localization and functionality upon expression in N. benthamiana L. plants. The recombinant enzyme resides in the Golgi as evidenced by the fluorescence distribution pattern of the GalNAc-T2:GFP fusion and alteration of the fluorescence signature upon treatment with Brefeldin A. A GalNAc-T2-specific acceptor peptide, the 113-136 aa fragment of chorionic gonadotropin β-subunit, is glycosylated in vitro by the plant-produced enzyme at the "native" GalNAc attachment sites, Ser-121 and Ser-127. Ectopic expression of GalNAc-T2 is sufficient to "arm" tobacco cells with the ability to perform GalNAc-glycosylation, as evidenced by the attachment of GalNAc to Thr-119 of the endogenous enzyme endochitinase. However, glycosylation of highly expressed recombinant glycoproteins, like magnICON-expressed E. coli enterotoxin B subunit:H. sapiens mucin 1 tandem repeat-derived peptide fusion protein (LTBMUC1), is limited by the low endogenous UDP-GalNAc substrate pool and the insufficient translocation of UDP-GalNAc to the Golgi lumen. Further genetic engineering of the GalNAc-T2 plants by co-expressing Y. enterocolitica UDP-GlcNAc 4-epimerase gene and C. elegans UDP-GlcNAc/UDP-GalNAc transporter gene overcomes these limitations as indicated by the expression of the model LTBMUC1 protein exclusively as a glycoform.

CONCLUSION

Plant bioreactors can be engineered that are capable of producing Tn antigen-containing recombinant therapeutics.

Phylogenetic and biochemical evidence supports the recruitment of an ADP-glucose translocator for the export of photosynthate during plastid endosymbiosis

The acquisition of photosynthesis by eukaryotic cells through enslavement of a cyanobacterium represents one of the most remarkable turning points in the history of life on Earth. In addition to endosymbiotic gene transfer, the acquisition of a protein import apparatus and the coordination of gene expression between host and endosymbiont genomes, the establishment of a metabolic connection was crucial for a functional endosymbiosis. It was previously hypothesized that the first metabolic connection between both partners of endosymbiosis was achieved through insertion of a host-derived metabolite transporter into the cyanobacterial plasma membrane. Reconstruction of starch metabolism in the common ancestor of photosynthetic eukaryotes suggested that adenosine diphosphoglucose (ADP-Glc), a bacterial-specific metabolite, was likely to be the photosynthate, which was exported from the early cyanobiont. However, extant plastid transporters that have evolved from host-derived endomembrane transporters do not transport ADP-Glc but simple phosphorylated sugars in exchange for orthophosphate. We now show that those eukaryotic nucleotide sugar transporters, which define the closest relatives to the common ancestor of extant plastid envelope carbon translocators, possess an innate ability for transporting ADP-Glc. Such an unexpected ability would have been required to establish plastid endosymbiosis.

Characterization of rice nucleotide sugar transporters capable of transporting UDP-galactose and UDP-glucose

Using the basic local alignment search tool (BLAST) algorithm to search the Oryza sativa (Japanese rice) nucleotide sequence databases with the Arabidopsis thaliana UDP-galactose transporter sequences as queries, we found a number of sequences encoding putative O. sativa UDP-galactose transporters. From these, we cloned four putative UDP-galactose transporters, designated OsUGT1, 2, 3 and 4, which exhibited high sequence similarity with Arabidopsis thaliana UDP-galactose transporters. OsUGT1, 2, 3 and 4 consisted of 350, 337, 345 and 358 amino acids, respectively, and all of these proteins were predicted to have multiple transmembrane domains. To examine the UDP-galactose transporter activity of the OsUGTs, we introduced the OsUGTs' expression vectors into UDP-galactose transporter activity-deficient Lec8 cells. Our results showed that transfection with OsUGT1, 2 and 3 resulted in recovery of the deficit phenotype of Lec8 cells, but transfection with OsUGT4 did not. The results of an in vitro nucleotide sugar transport assay of OsUGTs, carried out with a yeast expression system, suggested that OsUGT4 is a UDP-glucose transporter rather than a UDP-galactose transporter. Although plants have multiple UDP-galactose transporter genes, phylogenic analysis indicates that plant UDP-galactose transporter genes are not necessarily evolutionary related to each other.

The impact of the overexpression of human UDP-galactose transporter gene hUGT1 in tobacco plants

When the human UDP-galactose transporter 1 gene (hUGT1) was introduced into tobacco plants, the plants displayed enhanced growth during cultivation, and axillary shoots had an altered determinate growth habit, elongating beyond the primary shoots and having a sympodial growth pattern similar to that observed in tomatoes at a late cultivation stage. The architecture and properties of tissues in hUGT1-transgenic plants were also altered. The leaves had an increase in thickness, due to an increased amount of spongy tissue, and a higher content of chlorophyll a and b; the stems had an increased number of xylem vessels and accumulated lignin and arabinogalactan proteins (AGPs). Some of these characteristics resembled a gibberellin (GA)-responsive phenotype, suggesting involvement of GA. RT-PCR-based analysis of genes involved in GA biosynthesis suggested that the GA biosynthetic pathway was not activated. However, an increase in the proportion of galactose in polysaccharide side chains of AGPs was detected. These results suggested that because of higher UDP-galactose transport from the cytosol to the Golgi apparatus, galactose incorporation into polysaccharide side chains of AGP is involved in the gibberellin response, resulting in morphological and architectural changes.

The nucleotide sugar transporters AtUTr1 and AtUTr3 are required for the incorporation of UDP-glucose into the endoplasmic reticulum, are essential for pollen development and are needed for embryo sac progress in Arabidopsis thaliana

Uridine 5'-diphosphate (UDP)-glucose is transported into the lumen of the endoplasmic reticulum (ER), and the Arabidopsis nucleotide sugar transporter AtUTr1 has been proposed to play a role in this process; however, different lines of evidence suggest that another transporter(s) may also be involved. Here we show that AtUTr3 is involved in the transport of UDP-glucose and is located at the ER but also at the Golgi. Insertional mutants in AtUTr3 showed no obvious phenotype. Biochemical analysis in both AtUTr1 and AtUTr3 mutants indicates that uptake of UDP-glucose into the ER is mostly driven by these two transporters. Interestingly, the expression of AtUTr3 is induced by stimuli that trigger the unfolded protein response (UPR), a phenomenon also observed for AtUTr1, suggesting that both AtUTr1 and AtUTr3 are involved in supplying UDP-glucose into the ER lumen when misfolded proteins are accumulated. Disruption of both AtUTr1 and AtUTr3 causes lethality. Genetic analysis showed that the atutr1 atutr3 combination was not transmitted by pollen and was poorly transmitted by the ovules. Cell biology analysis indicates that knocking out both genes leads to abnormalities in both male and female germ line development. These results show that the nucleotide sugar transporters AtUTr1 and AtUTr3 are required for the incorporation of UDP-glucose into the ER, are essential for pollen development and are needed for embryo sac progress in Arabidopsis thaliana.

Intracellular metabolite transporters in plants

Due to the presence of plastids, eukaryotic photosynthetic cells represent the most highly compartmentalized eukaryotic cells. This high degree of compartmentation requires the transport of solutes across intracellular membrane systems by specific membrane transporters. In this review, we summarize the recent progress on functionally characterized intracellular plant membrane transporters and we link transporter functions to Arabidopsis gene identifiers and to the transporter classification system. In addition, we outline challenges in further elucidating the plant membrane permeome and we provide an outline of novel approaches for the functional characterization of membrane transporters.

Analysis of CMP-sialic acid transporter-like proteins in plants

It is commonly accepted that sialic acids do not exist in plants. However, putative gene homologs of animal sialyltransferases and CMP-sialic acid transporters have been detected in the genomes of some plants. To elucidate the physiological functions of these genes, we cloned 2 cDNAs from Oryza sativa (Japanese rice), each of which encodes a CMP-sialic acid transporter-like protein designated as OsCSTLP1 and OsCSTLP2. To examine the CMP-sialic acid transporter activity of OsCSTLP1 and OsCSTLP2, we introduced their expression vectors into CMP-sialic acid transporter activity-deficient Lec2 cells. Transfection with OsCSTLP1 resulted in recovery of the deficit phenotype of Lec2 cells, but transfection with OsCSTLP2 did not. We also performed an in vitro nucleotide sugar transport assay using a yeast expression system. Among the nucleotide sugars examined, the OsCSTLP1-containing yeast microsomal membrane vesicles specifically incorporated CMP-sialic acid, indicating that OsCSTLP1 has CMP-sialic acid transporter activity. On the other hand, OsCSTLP2 did not exhibit any nucleotide sugar transporter activity. T-DNA insertion lines of Arabidopsis thaliana targeting the homologs of the OsCSTLP1 and OsCSTLP2 genes exhibited a lethal phenotype, suggesting that these proteins play important roles in plant development and may transport important nucleotide sugars such as CMP-Kdo in physiological conditions.

The phosphate transporter PHT4;6 is a determinant of salt tolerance that is localized to the Golgi apparatus of Arabidopsis

Insertion mutations that disrupt the function of PHT4;6 (At5g44370) cause NaCl hypersensitivity of Arabidopsis seedlings that is characterized by reduced growth of the primary root, enhanced lateral branching, and swelling of root tips. Mutant phenotypes were exacerbated by sucrose, but not by equiosmolar concentrations of mannitol, and attenuated by low inorganic phosphate in the medium. Protein PHT4;6 belongs to the Major Facilitator Superfamily of permeases that shares significant sequence similarity to mammalian type-I Pi transporters and vesicular glutamate transporters, and is a member of the PHT4 family of putative intracellular phosphate transporters of plants. PHT4;6 localizes to the Golgi membrane and transport studies indicate that PHT4;6 facilitates the selective transport of Pi but not of chloride or inorganic anions. Phenotypic similarities with other mutants displaying root swelling suggest that PHT4;6 likely functions in protein N-glycosylation and cell wall biosynthesis, which are essential for salt tolerance. Together, our results indicate that PHT4;6 transports Pi out of the Golgi lumenal space for the re-cycling of the Pi released from glycosylation processes.

The import of S-adenosylmethionine into the Golgi apparatus is required for the methylation of homogalacturonan

S-adenosylmethionine (SAM) is the substrate used in the methylation of homogalacturonan (HGA) in the Golgi apparatus. SAM is synthesized in the cytosol, but it is not currently known how it is then transported into the Golgi. In this study, we find that HGA methyltransferase is present in Golgi-enriched fractions and that its catalytic domain faces the lumen of this organelle. This suggests that SAM must be imported into the Golgi. We performed uptake experiments using [methyl-(14)C]SAM and found that SAM is incorporated into the Golgi vesicles, resulting in the methylation of polymers that are sensitive to pectinase and pectin methylesterase but not to proteases. To avoid detecting the transfer reaction, we also used [carboxyl-(14)C]SAM, the uptake of which into Golgi vesicles was found to be sensitive to temperature, detergents, and osmotic changes, and to be saturable with a K(m) of 33 microm. Double-label uptake experiments using [methyl-(3)H]SAM and [carboxyl-(14)C]SAM also revealed a time-dependent increase in the (3)H to (14)C ratio, suggesting that upon transfer of the methyl group, the resulting S-adenosylhomocysteine is not accumulated in the Golgi. SAM incorporation was also found to be inhibited by S-adenosylhomocysteine, whereas UDP-GalA, UDP-GlcA, and acetyl-CoA had no effect. DIDS, a compound that inhibits nucleotide sugar transporters, also had little effect upon SAM incorporation. Interestingly, the combination of UDP-GalA + acetyl-CoA or UDP-GlcA + acetyl-CoA produced a slight increase in the uptake of SAM. These results support the idea that a SAM transporter is required for HGA biosynthesis.

Nucleotide-sugar transporters: structure, function and roles in vivo

The glycosylation of glycoconjugates and the biosynthesis of polysaccharides depend on nucleotide-sugars which are the substrates for glycosyltransferases. A large proportion of these enzymes are located within the lumen of the Golgi apparatus as well as the endoplasmic reticulum, while many of the nucleotide-sugars are synthesized in the cytosol. Thus, nucleotide-sugars are translocated from the cytosol to the lumen of the Golgi apparatus and endoplasmic reticulum by multiple spanning domain proteins known as nucleotide-sugar transporters (NSTs). These proteins were first identified biochemically and some of them were cloned by complementation of mutants. Genome and expressed sequence tag sequencing allowed the identification of a number of sequences that may encode for NSTs in different organisms. The functional characterization of some of these genes has shown that some of them can be highly specific in their substrate specificity while others can utilize up to three different nucleotide-sugars containing the same nucleotide. Mutations in genes encoding for NSTs can lead to changes in development in Drosophila melanogaster or Caenorhabditis elegans, as well as alterations in the infectivity of Leishmania donovani. In humans, the mutation of a GDP-fucose transporter is responsible for an impaired immune response as well as retarded growth. These results suggest that, even though there appear to be a fair number of genes encoding for NSTs, they are not functionally redundant and seem to play specific roles in glycosylation.

Characterization of AtNST-KT1, a novel UDP-galactose transporter from Arabidopsis thaliana

Nucleotide sugar transporters (NST) mediate the transfer of nucleotide sugars from the cytosol into the lumen of the endoplasmatic reticulum and the Golgi apparatus. Because the NSTs show similarities with the plastidic phosphate translocators (pPTs), these proteins were grouped into the TPT/NST superfamily. In this study, a member of the NST-KT family, AtNST-KT1, was functionally characterized by expression of the corresponding cDNA in yeast cells and subsequent transport experiments. The histidine-tagged protein was purified by affinity chromatography and reconstituted into proteoliposomes. The substrate specificity of AtNST-KT1 was determined by measuring the import of radiolabelled nucleotide mono phosphates into liposomes preloaded with various unlabelled nucleotide sugars. This approach has the advantage that only one substrate has to be used in a radioactively labelled form while all the nucleotide sugars can be provided unlabelled. It turned out that AtNST-KT1 represents a monospecific NST transporting UMP in counterexchange with UDP-Gal but did not transport other nucleotide sugars. The AtNST-KT1 gene is ubiquitously expressed in all tissues. AtNST-KT1 is localized to Golgi membranes. Thus, AtNST-KT1 is most probably involved in the synthesis of galactose-containing glyco-conjugates in plants.

AtUTr1, a UDP-glucose/UDP-galactose transporter from Arabidopsis thaliana, is located in the endoplasmic reticulum and up-regulated by the unfolded protein response

The folding of glycoproteins in the endoplasmic reticulum (ER) depends on a quality control mechanism mediated by the calnexin/calreticulin cycle. During this process, continuous glucose trimming and UDP-glucose-dependent re-glucosylation of unfolded glycoproteins takes place. To ensure proper folding, increases in misfolded proteins lead to up-regulation of the components involved in quality control through a process known as the unfolded protein response (UPR). Reglucosylation is catalyzed by the ER lumenal located enzyme UDP-glucose glycoprotein glucosyltransferase, but as UDP-glucose is synthesized in the cytosol, a UDP-glucose transporter is required in the calnexin/calreticulin cycle. Even though such a transporter has been hypothesized, no protein playing this role in the ER yet has been identified. Here we provide evidence that AtUTr1, a UDP-galactose/glucose transporter from Arabidopsis thaliana, responds to stimuli that trigger the UPR increasing its expression around 9-fold. The accumulation of AtUTr1 transcript is accompanied by an increase in the level of the AtUTr1 protein. Moreover, subcellular localization studies indicate that AtUTr1 is localized in the ER of plant cells. We reasoned that an impairment in AtUTr1 expression should perturb the calnexin/calreticulin cycle leading to an increase in misfolded protein and triggering the UPR. Toward that end, we analyzed an AtUTr1 insertional mutant and found an up-regulation of the ER chaperones BiP and calnexin, suggesting that these plants may be constitutively activating the UPR. Thus, we propose that in A. thaliana, AtUTr1 is the UDP-glucose transporter involved in quality control in the ER.